Description¶

These three functions add new static event sources to an event loop. The event loop object is specified in the event parameter, the event source object is returned in the source parameter. The event sources are enabled statically and will "fire" when the event loop is run and the conditions described below are met.

The handler is a function to call or NULL. The handler function will be passed the userdata pointer, which may be chosen freely by the caller. The handler may return negative to signal an error (see below), other return values are ignored. If handler is NULL, a default handler that calls sd_event_exit(3) will be used.

sd_event_add_defer() adds a new event source that will be dispatched instantly, before the event loop goes to sleep again and waits for new events. By default, the handler will be called once (SD_EVENT_ONESHOT). Note that if the event source is set to SD_EVENT_ON the event loop will never go to sleep again, but continuously call the handler, possibly interleaved with other event sources.

sd_event_add_post() adds a new event source that is run before the event loop will sleep and wait for new events, but only after at least one other non-post event source was dispatched. By default, the source is enabled permanently (SD_EVENT_ON). Note that this event source type will still allow the event loop to go to sleep again, even if set to SD_EVENT_ON, as long as no other event source is ever triggered.

sd_event_add_exit() adds a new event source that will be dispatched when the event loop is terminated with sd_event_exit(3).

The sd_event_source_set_enabled(3) function may be used to enable the event source permanently (SD_EVENT_ON) or to make it fire just once (SD_EVENT_ONESHOT).

If the handler function returns a negative error code, it will either be disabled after the invocation, even if the SD_EVENT_ON mode was requested before, or it will cause the loop to terminate, see sd_event_source_set_exit_on_failure(3).

To destroy an event source object use sd_event_source_unref(3), but note that the event source is only removed from the event loop when all references to the event source are dropped. To make sure an event source does not fire anymore, even when there's still a reference to it kept, consider setting the event source to SD_EVENT_OFF with sd_event_source_set_enabled(3).

If the second parameter of these functions is passed as NULL no reference to the event source object is returned. In this case the event source is considered "floating", and will be destroyed implicitly when the event loop itself is destroyed.

If the handler parameter to sd_event_add_defer() or sd_event_add_post() is NULL, and the event source fires, this will be considered a request to exit the event loop. In this case, the userdata parameter, cast to an integer, is passed as the exit code parameter to sd_event_exit(3). Similar functionality is not available for sd_event_add_exit(), as these types of event sources are only dispatched when exiting anyway.

Return Value¶

On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code.

Notes¶

Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file.

The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call setenv(3) from a parallel thread. It is recommended to only do calls to setenv() from an early phase of the program when no other threads have been started.

History¶

sd_event_add_defer(), sd_event_add_post(), sd_event_add_exit(), and sd_event_handler_t() were added in version 217.

See Also¶

systemd(1), sd-event(3), sd_event_new(3), sd_event_now(3), sd_event_add_io(3), sd_event_add_time(3), sd_event_add_signal(3), sd_event_add_child(3), sd_event_add_inotify(3), sd_event_source_set_enabled(3), sd_event_source_set_priority(3), sd_event_source_set_userdata(3), sd_event_source_set_description(3), sd_event_source_set_floating(3), sd_event_exit(3)